1. Field of the Invention
This invention relates generally to heterojunction bipolar transistors.
2. Discussion of the Related Art
A heterojunction is a type of semiconductor junction in which at least two adjacent regions are made of semiconductor materials with different bandgaps. For example, a heterojunction may have adjacent regions of silicon (Si) and strained silicon-germanium germanium (Si1xe2x88x92xGex), which have respective bandgaps of 1.12 eV and 1.2-0.7 eV, at room temperature. For a range of Ge molar fractions xe2x80x9cxxe2x80x9d, e.g., 0.1 less than x  less than 0.7, the adjacent Si and Si1xe2x88x92xGex, regions have different bandgaps and form a heterojunction.
In a heterojunction bipolar transistor (HBT), the difference in bandgaps strongly affects the transistor""s gain. In normal operating configurations, the transistor""s gain includes a factor of exp(xcex94Eg/kT) where xcex94Eg is the emitter bandgap minus the base bandgap, T is the temperature, and xe2x80x9ckxe2x80x9d is Boltzman""s constant. If the emitter bandgap is larger than the base bandgap, the above-described exponential factor enhances the transistor""s gain.
The factor exp(xcex94Eg/kT) dominates the transistor""s gain, at room temperature, if the difference between the emitter and base bandgaps is large, e.g., greater than 0.2-0.5 eV at 20xc2x0 Centigrade. This difference is very large for HBTs with Si emitters and Si1xe2x88x92xGex, bases if the molar Ge fraction xe2x80x9cxxe2x80x9d is greater than about 0.1.
Interest in Si/Sixe2x80x94Ge heterojunction devices increased as techniques for growing crystalline Sixe2x80x94Ge layers that such devices use increased. Presently, epitaxy techniques enable growing strained crystalline Sixe2x80x94Ge layers on crystalline Si substrates. See e.g., J. C. Bean et al, Appl. Phys. Lett. 44 (1983) 102-104. Nevertheless, progress is still needed to produce smaller Si/Sixe2x80x94Ge HBTs with high quality operating characteristics.
In one aspect, the invention features a process for forming a heterojunction device. The process includes forming one or more layers on a semiconductor substrate, forming a window in the layers to expose a portion of the substrate, and forming a silicon-germanium base region on the exposed portion of the substrate. The process also includes forming an emitter or collector region to cover the silicon-germanium base region, forming an oxide layer that covers the emitter or collector region, and forming a contact area on the emitter or collector region by removing a portion of the oxide layer.
In another aspect, the invention features a heterojunction bipolar transistor that includes an emitter or collector region of doped silicon, a base region including silicon-germanium, and a spacer. The emitter or collector region form a heterojunction with the base region. The spacer is positioned to electrically insulate the emitter or collector region from an external region. The spacer includes a silicon dioxide layer physically interposed between the emitter or collector region and the remainder of the spacer.